The present invention relates to processing orthogonal frequency division multiplexed (OFDM) signals.
Orthogonal frequency division multiplexing (OFDM) is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier.
The transmission of data through a channel via OFDM signals provides several advantages over more conventional transmission techniques. One advantage is the tolerance of OFDM to multipath delay spread. This tolerance is due to the relatively long symbol interval Ts compared to the typical time duration of the channel impulse response. These long symbol intervals prevent inter-symbol interference (ISI). Another advantage is the tolerance of OFDM to frequency selective fading. By including redundancy in the OFDM signal, data encoded onto fading sub-carriers can be reconstructed from the data recovered from the other sub-carriers. Yet another advantage is the efficient spectrum usage in OFDM. Since OFDM sub-carriers are placed in very close proximity to one another without the need to leave unused frequency space between them, OFDM can efficiently fill a channel. A further advantage is the simplified sub-channel equalization of OFDM. OFDM shifts channel equalization from the time domain (as in single carrier transmission systems) to the frequency domain where a bank of simple one-tap equalizers can individually adjust for the phase and amplitude distortion of each sub-channel. A still further advantage is the good interference properties of OFDM. It is possible to modify the OFDM spectrum to account for the distribution of power of an interfering signal. Also, it is possible to reduce out-of-band interference by avoiding the use of OFDM sub-carriers near the channel bandwidth edges.
Although OFDM exhibits these advantages, prior art implementations of OFDM also exhibit several difficulties and practical limitations. One difficulty is the issue of determining and correcting for carrier frequency offset, a major aspect of OFDM synchronization. Ideally, the receive carrier frequency, fcr, should exactly match the transmit carrier frequency, fct. If this condition is not met, however, the mis-match contributes to a non-zero carrier frequency offset, delta fc, in the received OFDM signal. OFDM signals are very susceptible to such carrier frequency offset which causes a loss of orthogonality between the OFDM sub-carriers and results in inter-carrier interference (ICI) and a severe increase in the bit error rate (BER) of the recovered data at the receiver.
Another difficulty is that of synchronizing the transmitter""s sample rate to the receiver""s sample rate to eliminate sampling rate offset. Any mis-match between these two sampling rates results in a rotation of the 2m-ary sub-symbol constellation from symbol to symbol in a frame for smaller frequency offsets. However, for larger frequency offsets, the result is a contraction or expansion of the frequency spectrum of the received signal. Both of these can contribute to increased BER. One cause of sampling rate offset is the presence of a sampling frequency offset. A sampling frequency offset occurs when the receiver samples the received signal at a frequency that is either higher or lower than the sample rate used at the transmitter. A sampling frequency offset can be detrimental to the performance of the receiver, and must be corrected for in order for the receiver to be properly synchronized. The present invention is directed to the correction of this problem.
An OFDM receiver samples an incoming signal in the time domain, multiplies the sampled data by a window function to lower the sidelobes of the frequency domain spectrum of each of the predetermined subcarriers in order to reduce interference between the predetermined subcarriers and also to widen the main lobe such that samples of sufficient magnitude (relative to the noise floor) will be available on either side of the main peak, takes an FFT (fast Fourier transform) of the sampled signal to analyze the frequency domain samples of each predetermined subcarrier, detects a difference in magnitude of the frequency domain samples on either side of each predetermined subcarrier, and generates a sampling frequency error based on the detected differences in magnitude.